The best dialog system has been around for a long time which is based on the standard dialog tree. Instead of exploring a dialog tree you have several descriptive options to start the conversation off and/or respond to the speaker and have a back and forth conversation while carfully choosing your response. It is much better then a timed system with one word choices that don't tell you exactly what you are going to say. I would also add the option to be able to speak to the npc afterwords and have a dialog tree to gather more background information and/or clues about the person or situation.

This way you can have exact knowledge on what you want to say, create the dialog flow for yourself, and be able to contemplate your choice. It also gives something for people (like me) who like to learn more about the background.

PS. I hope they did a better job at letting you know what you are going to say then Bioware did with Mass Effect since it is totally unrealistic to not know what you are going to say and is very immersion and roleplay breaking when you say something you didn't want to say because of lack of knowledge on what you are going to say.